Many different types of level sensors are known for sensing the level of a liquid in a tank or other container. Such level sensors are employed for use in storage tanks, the fuel tanks for automotive vehicles, and so forth. The most common type of level sensor employs a float type mechanism, whereby the float rides upon the surface of the liquid being monitored and is connected via an arm to a transducer. Typically, the transducer is a variable resistor for providing a voltage or current signal that varies in amplitude in proportion to the level of the liquid being monitored. The level signal is applied to a meter for giving a visual indication of the level of fuel in the tank. Such meters are commonly known as fuel or gasoline gauges in automotive vehicle use and are mounted on the dashboard where they can be easily monitored by a driver.
During the fuel crisis experienced in the 1970's, alternative fuels were developed and sold in the marketplace. Such fuels included mixtures of gasoline and methanol or gasoline and ethanol, for example. It would be useful to a vehicle operator, or a mechanic, to have a sensor providing an indication of the composition of fuel being used in the vehicle at any given time. Also, and more importantly, such a sensor can provide the engine computer of the vehicle with fuel composition data to control engine performance. Such a sensor might give a readout of the percent gasoline or percent ethanol or percent methanol, for example. Also, because of the use of different mixtures of gasoline and other fuel products, the resultant mixtures provide dielectric values that are different for each mixture and different than that of pure gasoline. These dielectric differences will cause errors in the readout of the level of fuel in the tank in level sensing systems including either capacitive or inductive type transducers for detecting the level of fuel, calibrated for gasoline, for example.
A number of known systems for measuring and providing a visual or other indication of the level of fuel in a tank will now be discussed. Lombard et al., U.S. Pat. No. 4,594,893, discloses a capacitive probe for measuring the level of liquid in a tank or pipe. The probe includes, as shown in the Figures, particularly FIGS. 1, 2, and 3, rectangular strip-like sections upon which are formed metallic strips for providing a reference capacitor on the lowermost strip, which is always immersed in liquid in a tank for serving as a reference capacitor and for further providing two parallel measuring capacitors identical in length and capacitance on an upper rectangular strip portion of the device for measuring liquid level within the tank. The reference capacitor is connected in parallel with one of the upper measuring capacitors to provide in combination a reference capacitance for the device, whereas the other measuring capacitor provides for a detectable changing capacitance with different liquid levels for permitting sensing of the liquid level. Electronic circuitry 23 (see FIG. 2) is only shown as a reference numeral, but not described.
Raymond et al., U.S. Pat. No. 4,571,543, teaches the use of interdigitated capacitors for measuring the concentration of a specific non-aqueous gas. As shown in the figures, the interdigitated capacitors 11 and 31 are juxtaposed on a rectangular substrate, whereby through the use of appropriate coatings permeable only to the gas of interest, for example, relative to one of the interdigitated capacitors, to permit measurement of changes in the capacitance caused by the gas that are proportional to the concentration of the gas. The other interdigitated capacitor provides for temperature sensing, whereby the combination of the capacitances of the two capacitors at any given time permit the presence and concentration of a specific gas to be determined. Note in FIGS. 3 and 5 that the interdigitated capacitors are connected in a circuit including an oscillator and balanced diode-quad bridges. The output of the diode-quad bridges are passed through low pass filters, the output of the filter or filters is amplified, and the amplified signal is read out from a digital indicator 61 (see FIG. 3) for indicating the presence and concentration of the particular gas. Note in FIG. 5 that a microprocessor is used for scanning a plurality of interdigitated capacitors connected through associated balanced diode-quad bridges, low pass filters, and DC amplifiers.
Hardway, U.S. Pat. No. 3,774,237, teaches a method and apparatus for detecting the moisture content or ingredients ratio of a material by measuring the difference in dielectric constant between the material and a reference material through the use of capacitive probes placed in close proximity to the reference material and sample material, whereby the differences in capacitive coupling measured between the probes and the materials is used in an electronic sensing circuit to produce an electrical output signal proportional to the difference in the dielectric constants between the samples. The output signal, which is an alternating signal, is rectified and provided as a measurable direct current signal for indicating the moisture content or percentage of a particular ingredient in the material.
In Mueller, U.S. Pat. No. 3,768,006, a capacitive probe is used for providing a means for measuring the capacitance of an oil-water emulsion, whereby in conjunction with an electronic detection network, the capacitance value is compared with the capacitance of a known capacitor for obtaining a signal indicative of the difference in capacitance therebetween to determine the percent water in the oil-water emulsion. Note in FIG. 4, a curve is shown relating the dielectric of the oil-water emulsion/dielectric of oil to the percent water content in the emulsion. Other figures, such as FIG. 3, show the curves relating to the dielectric of the emulsion to the percent water content of the emulsion.
Baum et al., U.S. Pat. No. 3,846,073, teaches the use of a capacitive sensor for measuring the capacitance of a copolymer fluid and comparing the value of this capacitance to a reference value for determining the composition of the copolymer.
Stoakes, U.S. Pat. No. 3,876,916, discloses capacitive probes of a particular design and associated detection circuitry for measuring the capacitance of a circulating fluid. Electronic detection circuitry includes a capacitance bridge for detecting the capacitance and applying a signal across a meter calibrated to accurately read the value of the parameter being measured.
Larson, U.S. Pat. No. 4, 389,889, teaches an apparatus for determining both the fuel level and the presence of water in the tank. Three individual plates are attached to the inside walls of the fuel tank. The plates are vertically oriented and juxtaposed to one another with the outer two of the plates being electrically connected together and the third plate located therebetween. An oscillator is connected to the two outermost plates for injecting an AC signal into the fluid in the tank. The signal from the oscillator is capacitively coupled to the center plate from the adjacent plates, and the level of the signal obtained from the center plate is detected, rectified, and supplied as a DC signal having an amplitude proportional to the level of the fluid to a meter 42 for providing a direct readout of the level. Also, another narrow strip-like plate 20 is located near the bottom of the tank, immediately below but separated from the previously mentioned center plate. The capacitance between the lowermost plate 20 and the center plate 14 is detected for determining when water is present in the tank. Since water is heavier than gasoline or diesel fuel, any water in the tank will drop to the bottom, and if sufficient in volume, will cover plate 20 causing a substantial increase in the signal level detected via plate 20, in turn causing a detection circuit to turn on a lamp indicating that the presence of water exceeds a predetermined level in the tank.
Yamanoue et al., U.S. Pat. No. 4,603,581, discloses a capacitive level sensing system for sensing the level of liquid in a vessel. In one embodiment, as shown in FIG. 8, Yamanoue teaches the use of a vertically oriented interdigitated capacitor across which a signal from an oscillator is connected, whereby the frequency of oscillation is determined by the value of capacitance measured, which is proportional to the level of fluid in the fuel tank. A sensing circuit for this configuration is not shown.
Atherton et al., U.S. Pat. No. 4,806,847, teaches the use of a capacitive probe for sensing the level of oil or transmission fluid in an engine. Signals developed across the probe and a reference capacitor are differentially detected with the detected output signal being rectified and provided as a signal proportional in level to the capacitance of the liquid or fluid being measured, which capacitance level is analogous to the level of the liquid.